Page 47 - Fundamentals of Physical Volcanology

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24 CHAPTER 2

lite are generated by melting of the crustal rocks
induced by heat transfer from the basaltic magma
ponded at the base of the crust. In other settings,
Arabian Peninsula
notably in the East African Rift Valley, a tremend-
ous diversity of compositions is found. The most
Red Sea
unusual magmas found in continental rift settings
Africa are carbonatites. Carbonatites are magmas which
Gulf of Aden contain greater than 50% carbonate minerals and
they have been observed being erupted at Oldoinyo
Lengai, a volcano in the East African Rift Valley.
East African Rift Afar Hot Spot were mudflows as they are jet black on eruption
Initial observers thought the carbonatite flows

and barely incandescent even when observed at
night. These lavas have the lowest eruption tem-

and the lowest viscosities of any known terrestrial
500 km peratures (typically 500–590°C at Oldoinyo Lengai)
lava. They behave like extremely fluid basalts (see
Fig. 2.7 Up-doming and faulting of the African continental Chapter 9).
crust by the Afar hot spot. Flood basalt volcanism dominates
the area (shaded area), and the Red Sea, the Gulf of Aden
and the East African Rift Valley form the three arms of a SUBDUCTION ZONES – ISLAND ARCS AND
triple junction. Sea-floor spreading has already started CONTINENTAL ARCS
along the Red Sea and the Gulf of Aden, but not in the East
Two types of subduction zone occur on Earth. In
African Rift Valley. (After fig. 3 in Ernst, R.E. and Buchan,
K.L. (1997) Giant radiating dyke swarms: their use in the first type, one oceanic plate subducts beneath
identifying pre-Mesozoic large igneous provinces and another oceanic plate generating an island arc; in
mantle plumes. Geophysical Monograph, 100, 297–333.) the second type an oceanic plate subducts beneath
a continental plate generating a continental arc
or active continental margin (Fig. 2.8). Island arcs
show a wide range of magma types, ranging from
basaltic magmas which dominate in these tectonic basalts through basaltic andesites and andesites to

settings are generated by partial melting of dacites and rhyolites. Although there is consider-
the mantle. able variation between different island arcs, the
dominant magma type is andesite, with basalts and
basaltic andesites being fairly common and the
CONTINENTAL INTRAPLATE SETTINGS
more evolved dacites and rhyolites being rarer.
These are settings which, like the MORs and Continental arcs show an even greater diversity of
oceanic hot spots, are zones of upwelling mantle magmas than island arcs and typically produce a
and so zones in which melting is likely to occur as greater proportion of evolved magmas than island
the result of decompression. As in the oceanic arcs. Although andesite is still a common composi-
setting, the initial interaction of a mantle plume tion, there are more dacites and rhyolites and less
with continental crust results in the production of basalts and basaltic andesites than are found in
flood basalts to form LIPs. After this initial phase, island arc settings.
magmatism continues generating basaltic magma but On the face of it, subduction zones present an
the interaction of these basalts with the continental unlikely setting for magma production because
crust creates a range of magma compositions. In they are zones in which the descent of cold oceanic
some instances, for example at Yellowstone, the lithosphere causes cooling of the surrounding
magmatism is bi-modal, producing both basaltic mantle, and in which mantle material is descending as
and rhyolitic magmas. The large volumes of rhyo- part of the large-scale convection system. So this is